The amygdala plays a critical role in the genesis of defensive behaviors. Moreover, it is hyperactive in humans afflicted with anxiety disorders. Thus, it is commonly believed that many anxiety disorders result, at least in part, from a dysregulation of amygdala processes normally mediating fear or defensive behaviors. Accordingly, research on the mechanisms controlling amygdala excitability might open new approaches for the treatment of anxiety disorders. This proposal aims to do just that, by studying the influence of midline thalamic (MTh) nuclei on the amygdala. Prior studies on thalamic influences over the amygdala have focused on inputs arising from the posterior thalamus, particularly from the medial portion of the medial geniculate nucleus. Yet, a number of tracing studies have revealed that MTh nuclei also contribute massive projections to the basolateral (BLA) and central (CeA) amygdala. However, other than anatomical data, little is known about the role of these strong glutamatergic inputs. The work proposed here aims to shed light on the influence of MTh inputs to the amygdala. To this end, we will first identify the targets and postsynaptic mechanisms of MTh inputs in the amygdala using anatomical (Aim #1) and physiological (Aim #2) methods. Indeed, BLA and CeA both contain multiple cell types that express different peptides/receptors and form contrasting connections with each other and extrinsic afferents. Therefore, in Aim #1, we will combine anterograde tracing with immunocytochemistry for various neuronal markers to identify the targets of MTh axon terminals in the amygdala at the light and electron microscopic levels. Building on these results, Aim #2 will combine optogenetic and patch clamp recording techniques in vitro to study the impact of MTh inputs on amygdala cells. Armed with this information, the last two aims will examine the influence of MTh cells on amygdala-dependent functions. Indeed, recent studies have revealed that following muscimol infusions in MTh nuclei, the expression of amygdala-dependent learned and innate fear is drastically reduced. However, it is unclear whether these muscimol findings result from the inhibition of nearby thalamic cells (e.g. mediodorsal nucleus), or the disfacilitaton of other targets of MTh nuclei (e.g. prefrontal cortex), that project to the amygdala. Two differen approaches will be used to address this question. First, in Aim #3, we will perform simultaneous extracellular recordings of MTh and amygdala cells during the expression of learned and innate fear. Next, In Aim #4, we will use a dual viral strategy allowing us to express halorhodopsin or channelrhodopsin, but only in MTh cells that project to the amygdala. We will then optogenetically inhibit or excite amygdala-projecting MTh cells and examine how this affects behavior on amygdala-dependent tasks that probe learned or innate fear. Together, the experiments proposed here will reveal how MTh neurons regulate the excitability of the amygdala during the expression of learned and innate fear. This knowledge will pave the way for pharmacological interventions aiming to regulate the activity of midline thalamic cells by taking advantage of their unusual profile of receptor expression.